How to place a .png texture with transparency over a material
The image texture node has two outputs, the color data and the alpha (transparency channel) data of the image.  The color data only holds the red, green, and blue of each pixel.  So if you plug it into a shader's color you will probably just get black where there is supposed to to transparency because those pixels are really black, but invisible.  You need to use the alpha output as a mask for placing the face over the skin shader.

To do this take whatever shaders you are using for the blank skin and mix them with a diffuse (or whatever other shader[s] you want for the facial features) based on the alpha output of the image texture.